1. Field of the Invention
This invention relates to a method of enhancing the nodulation ability and/or the competitive characteristics of strains of Rhizobium japonicum. More particularly, this invention relates to such a method in which said strains are incubated in the presence of soybean root exudate, soybean lectins, a composition comprising said lectin and/or root exudate or a combination thereof.
2. Description of the Prior Art
The process of biological nitrogen fixation consists of the reduction of elementary nitrogen to ammonia by living organisms. This biologically fixed nitrogen is the major source of nitrogen available for the formation of protein, which is essential in maintaining the world food supply. The economic importance of biological nitrogen fixation on the Earth cannot be over emphasized. It has been reported that nitrogen fixation by all biological organisms accounts for 80 percent of the nitrogen fixed on Earth per year. Leguminous and nonleguminous symbionts alone account for about 5.5 million tons of nitrogen fixed per year in the United States. About 13,000 species of leguminous plants exist, the vast majority of which are involved in a symbiotic relationship with nitrogen fixing bacteria. Some of the economically more important legumes include soybean, peas, clover and the like. As a result of increased world population there has been increased activity in enhancement of world food sources. One food crop which in recent years has received ever increasing emphasis is soybean because of its high protein content. Much of the enhanced protein content of soybean results from the superior nitrogen fixing capabilities of Rhizobium japonicum, the nitrogen fixing bacteria associated with soybean.
The establishment of a nitrogen fixing symbiosis in leguminous plants is a complex process involving physiological logical and biochemical properties of both the bacterium and host plant. The interaction of a particular legume species with its respective Rhizobium symbiont is known to be fairly specific. This has led to the establishment of cross-inoculation groups: that is, R. phaseolibean, R. trifolii-clover, R. meliloti-alfalfa, R. leguminosarum-pea, and R. japonicum-soybean. The underlying molecular mechanism to explain this infection specificity is presently an area of intense investigation. Lectins, proteins that bind carbohydrates, have been implicated as important in determining Rhizobium host specificity. According to this hypothesis, host-plants lectins located on the root surface recognize carbohydrate receptors on the compatible Rhizobium cell surface and, thereby, bind the bacteria to the root. This hypothesis was first proposed after demonstrating the specific binding of soybean lectin to Rhizobium japonicum but not to other incompatible Rhizobium species.
It has been demonstrated that nodulation on the roots of soybean and cowpea is developmentally restricted to the area between the root tip (RT) and smallest emergent root hair (SERH) observable with a dissecting microscope. Host root cells within this region become progressively less susceptible to nodulation as the root grows with successful nodulation occurring in this zone no later than four hours after inoculation. This time limit for the initiation of nodules within the RT-SERH zone enables one to infer the relative rate of infection by the position of the uppermost nodule as measured from the RT mark made at the time of inoculation.
To one skilled in the art, many R. japonicum strains of superior nitrogen fixation efficiency are available. Many of these strains, although efficient nitrogen fixers are of limited ability as they are unable to establish as nodule occupants in soil containing indigenous populations of rhizobia. The ability to improve the competitiveness to realize the maximum normal rhizobia is necessary to realize the maximum benefits of the Rhizobium symbiosis.
There is therefore a need to establish a method or composition which is capable of enhancing the nodulation ability and competitiveness of strains of Rhizobium japonicum.